This disclosure relates to data processing and storage, and more specifically, to techniques for selecting storage blocks in a data storage system, such as a flash memory system, for garbage collection based on longevity information.
NAND flash memory is an electrically programmable and erasable non-volatile memory technology that stores one or more bits of data per memory cell as a charge on the floating gate of a transistor or a similar charge trap structure. In a typical implementation, a NAND flash memory array is organized in blocks (also referred to as “erase blocks”) of physical memory, each of which includes multiple physical pages each in turn containing a multiplicity of memory cells. By virtue of the arrangement of the word and bit lines utilized to access memory cells, flash memory arrays can generally be programmed on a page basis, but are erased on a block basis.
As is known in the art, blocks of NAND flash memory must be erased prior to being programmed with new data. A block of NAND flash memory cells is erased by applying a high positive erase voltage pulse to the p-well bulk area of the selected block and by biasing to ground all of the word lines of the memory cells to be erased. Application of the erase pulse promotes tunneling of electrons off of the floating gates of the memory cells biased to ground to give them a net positive charge and thus transition the voltage thresholds of the memory cells toward the erased state. Each erase pulse is generally followed by an erase verify operation that reads the erase block to determine whether the erase operation was successful, for example, by verifying that less than a threshold number of memory cells in the erase block have been unsuccessfully erased. In general, erase pulses continue to be applied to the erase block until the erase verify operation succeeds or until a predetermined number of erase pulses have been used (i.e., the erase pulse budget is exhausted).
A NAND flash memory cell can be programmed by applying a positive high program voltage to the word line of the memory cell to be programmed and by applying an intermediate pass voltage to the memory cells in the same string in which programming is to be inhibited. Application of the program voltage causes tunneling of electrons onto the floating gate to change its state from an initial erased state to a programmed state having a net negative charge. Following programming, the programmed page is typically read in a read verify operation to ensure that the program operation was successful, for example, by verifying that less than a threshold number of memory cells in the programmed page contain bit errors. In general, program and read verify operations are applied to the page until the read verify operation succeeds or until a predetermined number of programming pulses have been used (i.e., the program pulse budget is exhausted).
A snapshot refers to creating a point-in-time data copy. With snapshot technology an original copy of data continues to be available to applications without interruption, while a snapshot is used to perform other functions on the data. Snapshots may be used by various applications, e.g., data protection, data analysis and reporting, and data replication applications, and facilitate efficient data protection. In general, snapshot technology facilitates improved application availability, faster recovery, easier back-up management of large volumes of data, reduced exposure to data loss, virtual elimination of backup windows, and lower total cost of ownership. Most leading storage hardware and software vendors implement snapshot technology. As one example, advanced data protection solutions such as IBM® Tivoli® Storage Manager implement snapshot technology.
Snapshots are generated to track changing blocks on a storage volume as writes to the storage volume are performed. With copy-on-write technology, original data that is to be overwritten is copied into a designated storage space that is set aside for a snapshot before the original data is overwritten. Typically, read requests to unchanged data blocks are redirected to the copied blocks in the snapshot, while read requests to active data blocks that have been changed are directed to the original storage volume. A snapshot includes metadata that describes changes to data blocks since the snapshot was first created. It should be noted that original data blocks are copied only once into the designated storage space for the snapshot, i.e., when the first write request is received for the original data blocks.